A number of unique refrigeration cycles and apparatus have been developed to satisfy the increasing demand for highly reliable, long-lasting cryogenic refrigerators for use in such diverse fields as electronic communications systems, missile tracking systems, super conducting circuitry, high field strength magnets, and medical and biology laboratories for preparation of tissue samples and freezing of solutions. These refrigeration cycles and apparatus, all based upon the controlled cycling of an expansible fluid with suitable heat exchange to obtain refrigeration, are exemplified by U.S. Pat. Nos. 2,906,101, 2,966,034, 2,966,035, 3,045,436, 3,115,015, 3,115,016, 3,119,237, 3,148,512, 3,188,819, 3,188,820, 3,188,821, 3,218,815, 3,333,433, 3,274,786, 3,321,926, 3,625,015, 3,733,837, 3,884,259, 4,078,389 and 4,118,943, and the prior art cited in the foregoing patents.
The present invention is directed at refrigeration systems which employ a working volume defined by a vessel having a displacer therein with a regenerator coupled between opposite ends of the vessel so that when the displacer is moved toward one end of the vessel, the refrigerant (working) fluid therein is driven through the regenerator to the opposite end of the vessel. Such systems may take various forms and employ various cycles, including the well known Gifford-McMahon, Taylor, Solvay and Split Stirling cycles. These refrigeration cycles and apparatus require valves or pistons for controlling the flow and movement of working fluid or the movement of the displacer means. The fluid flow and the displacer movement must be controlled continuously and accurately so that the system can operate according to a predetermined timing sequence as required by the particular refrigeration cycle for which the system is designed. Among the various types of valve systems that have been employed are rotary valves as exemplified by U.S. Pat. Nos. 3,119,237, 3,625,015, fluid actuated valves as shown in U.S. Pat. No. 3,321,926, cam operated valves as disclosed by U.S. Pat. No. 2,966,035, mechanically actuated slide valves as shown in U.S. Pat. No. 3,188,821, and displacer-operated valves as shown in U.S. Pat. No. 3,733,837.
Certain problems have been encountered in prior cryogenic systems because of the valving for the working fluid with the valving or the resulting refrigerator commonly being subject to one or more of the following limitations: complexity of construction, relatively high cost of manufacture, difficulty of modification as to timing sequence, and difficulty of adjustment after assembly. The problem of complexity in construction has been especially great where there have been attempts to achieve self-regulating refrigerators, i.e. refrigerators where the valving is operated by interaction with the displacer or by changes in fluid pressure produced directly or indirectly by the valving or the displacer valve systems. Additional specific limitations of prior cryogenic equipment have been excessive size of the valving (or of the refrigerator because of the valving construction and/or location), and reduced cooling efficiency due to limitations in valve designs.
U.S. Pat. No. 3,733,837 discloses self-regulating refrigerators in which cooling of a gas is achieved by expanding it in an expansion chamber, with gas flow to and from the expansion chamber being controlled by a valve having a slidable member operated by the displacer. The refrigerators are self-regulating in the sense that movement of the slidable valve member is controlled by the displacer and movement of the displacer is caused by a gas pressure differential determined by the position of the valve member. The refrigerators disclosed in U.S. Pat. No. 3,733,837 have a number of limitations. First of all the slide valves result in a relatively large void volume which is always filled with gas. Since the gas in the void volume is not cooled, the device has an efficiency limitation. The void volume can be reduced by reducing the diameter of the upper end of the displacer, but since that reduces the effective area it creates the adverse effect of reducing the pneumatic driving force on the displacer. On the other hand increasing the diameter of the upper end of the displacer, as may be desirable for larger capacity refrigerators, is troublesome since that cannot be done without proportionately increasing the overall size of the slide valve. Secondly, the fixed portion of the valve is located outside of the refrigeration cylinder while the movable valve member is located inside of the cylinder. Hence the valve does not lend itself to being preassembled as a discrete unit with precision-fitted parts.
Another prior slide valve design, disclosed in a copending U.S. Patent application of Domenic M. Sarcia for Cryogenic Apparatus, Ser. No. 89,274 filed Oct. 29, 1979 and intended for use in self-regulating refrigerators, has had the problem of tending to operate unreliably at low reciprocating speeds, e.g. below 5 cycles/sec., due to uneven loading of the slide valve member.